1. Field of the Invention
The invention relates to the field of rugate filters for reflecting selective electromagnetic frequencies and, in particular, to a method of making a multi-wavelength reflecting filter in the form of a coating on a translucent plastic or glass substrate.
2. Description of Related Art
It is well known in the field of optics that when light impinges upon any interface where there is a change in the index of refraction from one material to another, air to glass for example, some of the incident light will be reflected. In addition, at the interface where the light traverses from a material of relatively high index of refraction to one having a lower index of refraction, a phase change of 180 degrees occurs in the reflected light. Thus by properly selecting layer thickness, destructive cancellation of the incident light at consecutive interfaces is achieved. Consequently, the more interfaces an incident light beam traverses, the greater the amount of the incident light is reflected. Thus if enough layers are incorporated substantially all the light of a particular wavelength can be filtered out before reaching the substrate.
Therefore, conventional optical filters usually consist of a series of discrete layers of material deposited on an optical substrate. The material of each layer is selected, such that there is a change in the index of refraction at each interface, so that the index of refraction alternates from a higher value to a lower value or visa versa at each layer interface. Thus as the number of layers increases, the greater the amount of incident light that is reflected. Examples of "stacked" layer filter designs can be found in U.S. Pat. No. 5,238,738 "Polymeric Minus Filter" by R. H. Miller and U.S. Pat. No. 3,792,916 "Anti-Laser Optical Filter Assembly" by D. S. Sama. U.S. Pat. No. 3,853,783 "Vanadyl Phthalocyanine Sulfonamides And Laser Protective Plastic Filters Containing The Same" by R. J. Tucker particularly discloses a coating formulation offering protection form lasers operating from 620 to 720 nanometer wavelengths. However, such multi-layer coatings when used to protect from lasers operating over a wide range of discrete frequencies, become very thick. In addition, the occurrence of discrete interfaces makes the coatings relatively susceptible to mechanical failure and laser damage. Furthermore, the fabrication process does not lend itself to the non-periodic structure called for by multiple bands at non-commensurate wavelengths.
A newer approach is to use rugate filters that consist of a single layer of material formed wherein the index of refraction varies throughout its thickness. Because such rugate filters are typically formed by a continuous deposition process, it is an easy matter to vary the mixture deposited on the substrate, and thus vary the index of refraction. Examples can be found in U.S. Pat. No. 5,258,872 "Optical Filter" by W. E. Johnson, et al. and disclosed in U.S. Pat. No. 5,475,531 "Broadband Rugate Filter" by T. D. Rahminow, et al. They have the ability to meet optical specifications while avoiding the abrupt material interfaces inherent in a stacked layer filter. The option to implement multiple rejection bands within a common coating structure also tends to minimize mechanical failure issues.
It is also known that a number of wavelengths can be suppressed by having the profile of the final rugate filter be the resultant profile of the sum of the individual profiles desired, which is discussed in U.S. Pat. No. 5,523,882 "Rugate Filter Having Suppressed Harmonics" by T. D. Rahmolow. This patent discloses a concept for suppressing both the principle wavelength and its harmonics by setting the index of refraction versus optical thickness profile that superimposes on the principle sinusoid for the rejection of a principle wavelength a secondary sinusoid having a index of refraction versus optical thickness profile with an amplitude of about 10 percent of the principle sinusoid, and a phase difference of about minus 90 degrees. U.S. Pat. No. 5,293,548 "Dispersive Rugate Coatings" by E. T. Siebert also discloses coatings having a spatially varying index of refraction profile through the depth thereof so as to provide a prescribed dispersion characteristic that matches a dispersion characteristic of a source of radiation signal. Thus rugate filters are well known in the art.
However, there are several problems in making such rugate filters, among the most important is that the prior art processes for making such filters typically depend upon processes that required expensive vacuum deposition techniques commonly called "sputtering." In the sputtering process it is difficult to accurately control the sputtering of two materials to precisely vary the index of refraction. Other processes such as laser flash evaporation, ion beam assisted deposition, resistive and electron-beam evaporation all do not lend themselves to large plastic or glass substrates and/or require relatively expensive equipment. The use of vapor deposition processes eliminates some of the difficulties encountered when sputtering. But providing a process that accurately varies the index of refraction so as to provide precise reflectance bands, while providing maximum light transmittance therethrough, without requiring expensive equipment and/or process control procedures has not been heretofore available.
Thus, it is a primary object of the invention to provide a process for making a rugate filter having multiple reflectance bands.
It is another primary object of the invention to provide a process for making a rugate filter having multiple reflectance bands and improved transmittance without requiring expensive equipment.
It is a further object of the invention to provide a process for making a rugate filter with an index of refraction profile that simulates a continuously varying refractive index by depositing discrete micro-layers.